1. Field of the Invention
The present invention relates to a process for separating silver for recovery and purification by extraction from solids containing slightly soluble silver compounds which occur during smelting of noble metals or during disposal of wastes of plating solution and developing solution.
2. Description of the Related Art
One typical way of recovering silver from aqueous solutions or solids by hydrometallugical process is by conversion of silver into a slightly water-soluble silver compound such as silver chloride. This process offers the advantage of being able to separate and recover silver from most coexisting metals including noble metals. Unfortunately, the recovered silver compound (in the form of chloride or sulfate) is difficult to purify directly by hydrometallugical process because it is only slightly soluble not only in water but also in acid and alkali.
There are some known processes for separating such slightly soluble silver compounds from coexisting other compounds or solids. They include the ammonia leaching process and thiosulfate leaching process by which silver is dissolved in the form of stable complex compounds. Unfortunately, the ammonia leaching process has the disadvantage that silver ions react with ammonia to give unstable explosive silver compounds such as silver fulminate when the solution is allowed to stand. This prevents the leachate from residing or being recycled in the system. On the other hand, the thiosulfate leaching process tends to precipitate slightly soluble silver sulfide when the leachate is allowed to stand. This tendency is strong when the leachate decreases in pH and increases in temperature. An additional disadvantage is that metallic silver recovered from the leachate by electrolysis or reduction contains much sulfur.
Moreover, the above-mentioned two leaching processes involve a problem in common. The problem is that many other complex-forming elements are leached out together with silver during leaching. Consequently, the silver recovered from the leachate by reduction contains these elements and needs repurification by pyrometallugical process or electrolysis. This object is achieved only by the aid of a strong reducing agent because silver exists in the form of stable complex ions in the leachate. The use of a strong reducing agent leads to an increased production cost and poses a problem with the accumulation of reducing agent or decomposed products thereof in the leachate, which makes it difficult to repeatedly use the stripping solution after the silver recovery. On the other hand, the collection by electrolysis needs additional equipment and increases the residence time for silver. In addition, it prevents the leaching solution from being recycled on account of decomposition by anodic oxidation.
There is another reducing process applicable to the ammonia leachate which employs hydrogen at a high temperature under a high pressure. This process, however, is not of practical use because of the possibility of explosion by hydrogen and the danger of forming explosive silver compounds (such as silver fulminate) as mentioned above.
In the meantime, it is known that an organophosphorus compound containing in the molecule sulfur in the form of S.sup.2- ions extracts silver ions from an aqueous solution of silver. In practical application, it is necessary to dissolve silver (contained in solids) in nitric acid to give an aqueous solution of silver nitrate. Unfortunately, nitric acid, however dilute it might be, easily degrades (by oxidation) the above-mentioned organophosphorus compound. Therefore, this process cannot be used to recover silver from solids an industrial scale.